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If the light is a wave, how does one explain the rectilinear propagation of light? This is related to how the diffraction pattern vanishes and light cast sharp shadows on the screen. An appreciable bending/deviation of light across the edges of the slit in single-slit diffraction requires a narrow slit. But I don't find a satisfactory explanation anywhere.

I have tried to understand this by imagining a wide slit to be made up of a series of a large number of narrow slits but failed to explain why there is more bending for narrow slits.

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    $\begingroup$ Possible duplicate of If light rays obey to the wave equation, why can they be thought as straight lines?. If this doesn't answer your question, you should edit this post to explain why. $\endgroup$
    – Emilio Pisanty
    Commented Aug 8, 2017 at 19:21
  • $\begingroup$ @EmilioPisanty already expanded my question. I'm looking for a more physical explanation. My question is why light bends more when the slit is narrow and propagates almost in a rectilinear fashion when the slit is wide. $\endgroup$
    – Solidification
    Commented Aug 8, 2017 at 19:25
  • $\begingroup$ See the two chapters on Interference and Diffraction in Feynman's lectures (Vol. I). $\endgroup$
    – NickD
    Commented Aug 8, 2017 at 20:17

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According to the Huygens-Fresnel principle, every point on a wavefront is a source of wavelets. When the front is wide, the wavelets add up in such a way that the front propagates forward in the same shape, except for diffraction at the edges. However, when the front is small and becomes comparable in size with the wevelength of light, the slit essentially becomes a new source of a wave that expands from there in all directions.

https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle

In a space with an odd number of dimensions, such as our 3D space, the back wave is canceled. For this reason a light beam of a flashlight does not reflect back from the empty space to blind you. It would do so in a space with an even number of dimensions. You can see on a surface of water that the back wave is not canceled. Light from a flash leaves the darkness behind, but a wave from a rock thrown in a lake expands while also remaining in the center.

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